1. Technical Field
The present application relates to a nitride semiconductor light emitting element and an LED system including a plurality of nitride semiconductor light emitting elements.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a group V element is a prime candidate for a material to make a short-wavelength light emitting element because of its wide bandgap. Among other things, gallium nitride-based compound semiconductors have been researched and developed particularly extensively. As a result, blue light emitting diodes (LEDs), green LEDs, and blue semiconductor lasers, which use the gallium nitride-based compound semiconductors, have already been used in actual products.
Nitride semiconductors include compound semiconductors that substitute at least one of aluminum (Al) and indium (In) for some or all of gallium (Ga). Such nitride semiconductors are expressed by a composition formula of AlxGayInzN (0≦x, y, z≦1, x+y+z=1). In the following, the gallium nitride-based compound semiconductors are referred to as nitride semiconductors.
By replacing Ga atoms with Al atoms, the bandgap can be greater than that of GaN. By replacing Ga atoms with In atoms, the bandgap can be smaller than that of GaN. In this manner, not only nitride semiconductor elements that emit short-wavelength light such as blue light or green light but also nitride semiconductor elements that emit orange light or red light are obtained. Due to these characteristics, a nitride semiconductor light emitting element is also expected to be applied to an image display device or an illumination device.
Nitride semiconductors have a wurtzite crystal structure. FIGS. 1A, 1B, and 1C illustrate planes of the wurtzite crystal structure expressed in four-digit indices (hexagonal indices). Four-digit indices use basis vectors denoted by a1, a2, a3, and c to express crystal planes and orientations. The basis vector c runs in a [0001] direction, which is called “c-axis”. A plane perpendicular to the c-axis is called “c-plane” or “(0001) plane”. FIG. 1A illustrates not only the c-plane but also an a-plane and an m-plane. Further, FIG. 1B illustrates an r-plane, and FIG. 1C illustrates a (11-22) plane.
FIG. 2A illustrates a crystal structure of a nitride semiconductor represented by a ball-and-stick model. FIG. 2B illustrates an atomic arrangement in the vicinity of a surface of the m-plane observed from an a-axis direction. The m-plane is perpendicular to the drawing sheet of FIG. 2B. FIG. 2C illustrates an atomic arrangement on a surface of a +c-plane observed from an m-axis direction. The c-plane is perpendicular to the drawing sheet of FIG. 2C. As can be seen from FIG. 2B, N atoms and Ga atoms are located on a plane in parallel with the m-plane. On the other hand, with regard to the c-plane, as can be seen from FIG. 2C, a layer in which only Ga atoms are placed and a layer in which only N atoms are placed are formed.
Conventionally, when manufacturing a semiconductor element from a nitride semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as a principal surface is used as a substrate on which a nitride semiconductor crystal is to be grown. In this case, because of this arrangement of Ga atoms and N atoms, spontaneous electrical polarization occurs in the nitride semiconductor in the c-axis direction. The “c-plane” is therefore also called “polar plane”. As a result of the electrical polarization, a piezo-electric field is generated along the c-axis direction in a quantum well of InGaN in a light emitting layer of the nitride semiconductor light emitting element, and shifts the distribution of electrons and holes within the light emitting layer, thereby lowering the internal quantum efficiency of the light emitting layer through quantum-confined Stark effect of carriers, which has been a problem. In order to inhibit lowering of internal quantum efficiency of the light emitting layer, the thickness of the light emitting layer formed on the (0001) plane is designed to be 3 nm or less.
Further, in recent years, manufacturing a light emitting element using a substrate having, as a surface, the m-plane or the a-plane which are called non-polar planes, or a −r-plane or the (11-22) plane which are called semi-polar planes is under study. As illustrated in FIG. 1, m-planes in the wurtzite crystal structure are parallel to the c-axis and are six equivalent planes orthogonal to the c-planes. For instance, a (1-100) plane perpendicular to a [1-100] direction in FIG. 1 is an m-plane. Other m-planes equivalent to the (1-100) plane include a (-1010) plane, a (10-10) plane, a (- 1100) plane, a (01-10) plane, and a (0-110) plane. The sign “−” to the left of a number inside the parentheses that indicates a Miller index means a “bar”.
As illustrated in FIG. 2B, Ga atoms and N atoms on an m-plane exist on the same atomic plane, and therefore, electrical polarization does not occur in a direction perpendicular to the m-plane. Accordingly, using a semiconductor multilayer structure that is formed on an m-plane in the manufacture of a light emitting element prevents the generation of a piezo-electric field in the light emitting layer, thereby solving the problem of lowering the internal quantum efficiency which is caused by quantum-confined Stark effect of carriers. The same can be said with regard to the a-plane which is a non-polar plane other than the m-plane, and a similar effect can be obtained with regard to the −r-plane or the (11-22) plane which are called semi-polar planes.